EMC Testing and monitor

Guest
Hi,

Please do not hate me or make remarks about my lack of knowledge about EMC. I am doing some research to learn some basics about EMC testing.

I found the following report on EMC testing of a PC monitor

http://www.bricomp.net/msia/app/data/misc/LBE073289_MY17LS%28Mckinley_743N%29_FCC%28AUO,%20SEMCO%29.pdf


I have following questions

1. This montior did pass the EMC testing. The test report shows two graphs on pages 15 and 16 of the report 9 (Level vs Frequency) . Can someone give me some pointers on how to read and understand this graph?

I am unable to understand what is the maximum value dB(uv /m) that is not acceptable by FCC? It seems like that the tester put the limit to 30uV/m. But why this number. Is 30uV/m standard for monitors and could be different for lets say key board or mouse?


2. What is the difference between horizontal polarization and vertical polarization?

melissa
 
Hi,
Thanks for your reply. Can you also advise me on the following issues?

I am unable to understand what is the maximum value dB(uv /m) that is not acceptable by FCC? It seems like that the tester put the limit to 30uV/m in the report. But why this number. Is 30uV/m standard for monitors and could be different for lets say key board or mouse?

melissa
 
<walravenmelissa@gmail.com> wrote in message
news:f7ccc0e7-e042-420f-b180-bb3f34692f3a@googlegroups.com...
Hi,

Please do not hate me or make remarks about my lack of knowledge about EMC.
I am doing some research to learn some basics about EMC testing.

I found the following report on EMC testing of a PC monitor

http://www.bricomp.net/msia/app/data/misc/LBE073289_MY17LS%28Mckinley_743N%29_FCC%28AUO,%20SEMCO%29.pdf


I have following questions

1. This montior did pass the EMC testing. The test report shows two graphs
on pages 15 and 16 of the report 9 (Level vs Frequency) . Can someone give
me some pointers on how to read and understand this graph?

I am unable to understand what is the maximum value dB(uv /m) that is not
acceptable by FCC? It seems like that the tester put the limit to 30uV/m.
But why this number. Is 30uV/m standard for monitors and could be different
for lets say key board or mouse?


2. What is the difference between horizontal polarization and vertical
polarization?

melissa
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

The FCC sets standards mostly so the devices will not cause any interferance
with other electronic devices, mostly radio and TV receivers. On the graphs
the horizontal axis is the frequency . Higher frequencies to the right.
The vertical is in microvolts per meter. This is a way to tell how strong
the signal is. The graph increments in decibels. Each 10 DB is 10 times as
strong. Starting at 0 DB means you have one microvolt (uV) per meter. At
the 10 mark it is 10 times as strong, 20 DB is 100 times as strong, 30 db
1000 times as strong.

Many devices radiate radio frequencies that cause problems with other
receivers. While you never reach true zero level, the greater the distance
the weaker it gets. If you start off at a low level, then hopefully other
receiving devices will be far enough away as not to cause problems. Look at
it this way, you have an FM radio and want to use it next to the computer.
While on the same desk, the radiation from the computer may over ride the FM
broadcast station. You move the receiver across the room and the signal
from the computer is weak enough youcan receive the FM station.

As to horizontal and vetical polarization, that is sort of easy. That is in
relationship to the earth. If an antenna is parallel to th earth like the
old outside antennas, that is horizontal and if the antenna is vertical like
most AM radio stations, that is vertical.
 
On Sun, 08 Mar 2015 21:47:11 -0700, <walravenmelissa@gmail.com> wrote:

Hi,
Thanks for your reply. Can you also advise me on the following issues?

I am unable to understand what is the maximum value dB(uv /m) that is
not acceptable by FCC? It seems like that the tester put the limit to
30uV/m in the report. But why this number. Is 30uV/m standard for
monitors and could be different for lets say key board or mouse?

melissa
Click to expand...

The numbers are SET by the Regulatory Agency, in this case FCC.

BUT, to prevent confusion emanations from electronic equipment are
measured in a specific way. Procedures, time to measure, bandwidths etc
etc are all specified.

But for simplicity, envision an Open Air Test Site, OATS for short, with a
metal Ground plane covering a huge area. Then using what is called a
Robert's Antenna, [Robert's Antenna is like a set of completely opened
rabbit ears] which is a dipole antenna with its length adjusted (tuned)
for the frequency of interest, at a distance of 30 meters sometimes 100
meters, you measure what comes OFF equipment with the equipment sitting on
a non-conductive bench approx 1 meter above the Ground plane. [if the
equipment is BIG, it simply sits isolated from the ground plan, as in
giant set of rack mounted equipment]

To speed up the test, FCC allows you to use a set of non-adjustable,
BroadBand Antennas, and 'adjust' the receiver's readings with calibration
values, BUT Robert's Antenna always takes precednece.

Obviously, a dipole antenna has polarization. When the 'arms' are vertical
to the ground plane, called vertical polariztion. When horizontal, called
horizontal polarization. The antenna must also be held anywhere from 1
meter above the plane to ?? forgot is that 12 meters or 10 meters above
the plane? Note doesn't always work well because at low frequency when the
Robert's antenna arms are extended, the lower arm hits the plane, so got
to'fudge' around a bit. To add insult to injury, FCC requires the antenna
be moved to where you get the largest signal.

From memory, FCC has set values for maximum emanations measured at 30
meter distance [and 100 meters distance] from the Equipment. THEY set the
values after prolonged arguments. Two permissable levels exist. Class A
which is assumed to go into Industrial locations so is less stringent than
Class B which is assumed to be in residential areas. Note the difference
in distance means the receiver is operating in relatively the same
reception level for Class A an for Class B.

At distance of 30 meters the signals can be very small, so FCC allows you
to move the antenna system closer, and then you 'adjust' the reading as
though they were taken at 30 meters. The advantage of being closer is that
nearby radio stations, FM stations, and TV stations signals appear
relatively smaller. Now, to AVOID confusiion ALL reports are written in
terms of 'what if' the measurements were taken at 30 meters, adjusted so
when anyone looks at a report they can compare apples to apples, AND the
limits can be drawn in, and therefore always the same.

There are a lot of nuances in taking EMC measurements. But, basically, the
OATS method is still the 'standard'.

Do a search for OATS and anything written circa 80's to get the basics.
Perhaps, HP, now Agilent, or Rhode & Scwartz(sp?) have some basic
information in their AppNotes at their websites.
 
Hi,

So, 30uV/m limit is set by FCC for all the EMC testing for computer monitors! A monitor will not be approved if it generate noise more than 30uV/m. Am I right?

melissa
 
On Mon, 09 Mar 2015 15:04:34 -0700, <walravenmelissa@gmail.com> wrote:

Hi,

So, 30uV/m limit is set by FCC for all the EMC testing for computer
monitors! A monitor will not be approved if it generate noise more than
30uV/m. Am I right?

melissa
Click to expand...

you need to get a copy of the FCC rules to read tables of limits, read the
description os how equipment is categorized, see the plots of those limits.

From memory, FCC Part 15, relating to electronics and unintentiaonal
radiators.


I have seen some monitors that cost on the order of $40,000 [Silicon
Grpahics?] that due to their price were categorized as Class A, very
unlikely to end up in people's homes. Thus their limits were higher.
 
On Mon, 09 Mar 2015 15:04:34 -0700, <walravenmelissa@gmail.com> wrote:

Hi,

So, 30uV/m limit is set by FCC for all the EMC testing for computer
monitors! A monitor will not be approved if it generate noise more than
30uV/m. Am I right?

melissa
Click to expand...

I didn't have to look the FCC numbers up, limit at what distance, they're
in your report!

No, you are NOT right. Depends on what frequency you're looking at. See
your report.
page 13/17 shows the limits for Class B
30 to 230MHz limit is 30dBuV/m
230 to 1000MHz limit is 37 dBuV/m

Plus, that's a well written report, describes what, how, and with what
they tested the LCD Monitor, even great pictures showing those two
Schaffner Biconical Antennas!

For what it's worth, Those test results show a measurement fairly close to
the limit AND then clearly states that the measurement uncertainty is MORE
than that margin. Could easily move the signal 'out of compliance' if
literlly applied.

That's one of the reasons Sony's consumer division has their internal
margin enforced with an 8 dB margin!! In other words the FCC specification
says one limit, but everything you measure in the test lab had better be
more than 8 dB BELOW that limit! Not easy to do either.
 
On Sun, 8 Mar 2015 20:25:01 -0700 (PDT), walravenmelissa@gmail.com
wrote:

Hi,

Please do not hate me or make remarks about my lack of knowledge about EMC. I am doing some research to learn some basics about EMC testing.

I found the following report on EMC testing of a PC monitor

http://www.bricomp.net/msia/app/data/misc/LBE073289_MY17LS%28Mckinley_743N%29_FCC%28AUO,%20SEMCO%29.pdf


I have following questions

1. This montior did pass the EMC testing. The test report shows two graphs on pages 15 and 16 of the report 9 (Level vs Frequency) . Can someone give me some pointers on how to read and understand this graph?

I am unable to understand what is the maximum value dB(uv /m) that is not acceptable by FCC? It seems like that the tester put the limit to 30uV/m. But why this number. Is 30uV/m standard for monitors and could be different for lets say key board or mouse?


2. What is the difference between horizontal polarization and vertical polarization?
Click to expand...

Think of an electromagnetic wave as an eel swimming through the water.
Some eels may squirm sideways to swim and some eels may prefer to
squirm up and down. So the e-field, the voltage part of the wave, may
be horizontal or vertical, and a dipole antenna has to be oriented to
couple to the field.

Various parts of the monitor might radiate at various angles, not
necessarily horizontal or vertical (imagine drunken eels) but one
horizontal antenna and one vertical antenna will catch anything.

The h-field, the magnetic wave, is at right angles to the e-field. The
e-field creates the h-field, and the h-field creates the e-field, and
they squirm away at the speed of light.




--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
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